1. Field of the Invention
The present invention relates to a betatron, particularly to the production of x-radiation in an x-ray inspection system, with a yoke which guides the magnetic flux and includes at least partially of a composite powder.
2. Description of the Background Art
X-ray inspection systems such as the one illustrated in FIG. 2 are used, as is well-known, in the inspection of large-volume articles such as containers and motor vehicles for illegal contents such as weapons, explosives, or contraband goods. In so doing, x-radiation is produced and directed at the article (e.g., target 50). The x-radiation attenuated by the object is measured by means of a detector (e.g., x-ray detector 52) and analyzed by an evaluation unit (e.g., evaluation unit 54). Therefore, a conclusion can be reached on the nature of the object. This type of x-ray inspection system is known, for example, from European Pat. No. EP 0 412 190 B1, which corresponds to U.S. Pat. No. 5,065,418.
Betatrons are used to generate x-radiation with the energy of more than 1 MeV needed for the inspection. These are circular accelerators in which electrons are held in an orbit by a magnetic field. A change in this magnetic field produces an electric field, which accelerates the electrons in their orbit. A stable nominal orbit radius is determined from the so-called Wideroe condition depending on the course of the magnetic field and its change with time. The accelerated electrons are guided onto a target, where upon impacting they produce Bremsstrahlung whose spectrum depends, inter alia, on the energy of the electrons.
A betatron disclosed in Offenlegungsschrift [Unexamined German Pat. Application] No. DE 23 57 126 A1 includes a two-part inner yoke, in which the front sides of both inner yoke parts face each other spaced apart. A magnetic field is produced in the inner yoke by means of two main field coils. An outer yoke connects the two inner yoke part ends distant from one another and closes the magnetic circuit.
An evacuated betatron tube, in which the electrons to be accelerated circulate, is arranged between the front sides of the two inner yoke parts. The front sides of the inner yoke parts are formed in such a way that the magnetic field produced by the main field coil forces the electrons into a circular orbit and moreover focuses them onto the plane in which this orbit lies. To control the magnetic flux, it is prior in the art to arrange a ferromagnetic insert between the front sides of the inner yoke parts within the betatron tube.
In prior-art known betatrons, the yokes include laminated cores, which are formed particularly of transformer sheets. In this respect, the inner yoke in particular must be fabricated very precisely to achieve the greatest possible homogeneity of the magnetic field in the region of the betatron tube. The manufacture of the yokes from laminated cores is therefore complex and expensive, and, moreover, cracks often result during the lamination of the sheets. A mechanical finishing of the laminated cores results in a “smearing” of the surface, which leads to increased eddy current losses during operation. Cleaning of the surface, for example, by an etching process is a conventional procedure to remove this layer, but disadvantageous for reasons of environmental protection and occupational safety.